In any Electrocardiogram (ECG) monitoring device, an important feature is the detection and characterization of each individual heart beat present in the ECG signal. This information is then used to generate both heart rate information and alarms in life threatening situations. Monitoring an ECG signal from a patient having a pacemaker is difficult as pace pulses generated by the pacemaker can occur at any time. When they occur between QRS complexes they can be incorrectly detected by a QRS detector and result in an incorrect high heart rate measurement. When they occur during a QRS complex, they can cause incorrect feature measurement and result in an erroneous QRS classification. In particular, the detection of asystole is necessary to alert nurses of the cessation of heart activity which is indicated by the absence of the QRS complex in the ECG signal. However, in the case of patients with pacemakers, the ECG signal, even after asystole, contains periodically occurring pace pulses, which may resemble heart activity. The presence of pace pulses on an ECG signal makes it difficult to detect such asystole conditions.
A typical pace pulse consists of two components, a main pulse and a repolarization pulse. The main pulse, which is used to stimulate the heart, is characterized by its narrow width, sharp rise and fall, and large variation in amplitude. The actual shape of the pace pulse depends on the output coupling design of the pacemaker. The repolarization pulse, sometimes referred to as a pace pulse tail, is used to deplete the capacitive coupling generated by the delivery of the pace pulse charge built up between the heart and the pacemaker. The shape and size of the pace pulse tail is a function of the energy content of the pace pulse and the amount of capacitive coupling. In addition to repolarization, bandpass filtering in the monitoring equipment may create a "pace pulse tail".
Two examples of pace pulse signals recorded on the surface ECG are shown in FIGS. 1A and 1B. FIG. 1A is a pace pulse with a small repolarization tail, whereas FIG. 1B illustrates a large repolarization tail generated by the pacemaker system. As shown in FIGS. 1C, and 1D, both pace pulse signals exhibit significant repolarization tails after bandpass filtering.
In order to more accurately monitor ECG signals it has been found helpful to eliminate pace pulse signals. Such elimination requires that the pace pulse first be identified. The process of identifying pace pulses may employ the technique disclosed in U.S. Pat. No. 4,664,116 and incorporated by reference; wherein, pace pulses are identified by the existence of high frequency "spikes" having narrow width and a sharp rise time which exceeds a minimum dynamic noise threshold.
Additional hardware and software can be employed to remove detected pace pulses (FIG. 3). In particular, a technique is described in U.S. Pat. No. 4,832,041 in which values of the ECG signal that are within a window containing the pace pulse are replaced with substitute values that are an interpolation of selected values of the ECG signal. The substitute values form a line that is very close to what the ECG signal would be if a pace pulse had not occurred. However, this algorithm is not designed to eliminate the pace pulse tail. FIG. 2A shows pace pulse signals with the pace pulse tail, and FIG. 2B illustrates the pace pulse tail after the pace pulse spike has been removed using the above mentioned technique. Unfortunately, the remaining energy of the pace pulse tail may be erroneously detected as a QRS complex. This may cause the misdiagnosis of the patient's underlying ECG rhythm and result in a missed detection of an asystole condition.
Accordingly it is the purpose of this invention to provide a method for differentiating pace pulse tails from true QRS complexes in an ECG signal waveforms.